Magnetic Resonance Imaging (MRI) has become a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structure of objects (such as the human body) having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR) phenomena. In MRI, nuclei in a body to be imaged are polarized by imposing a strong main magnetic field Ho on the nuclei. Selected nuclei are excited by imposing a radio frequency (RF) signal at a particular NMR frequency. By spatially distributing the localized magnetic fields, and then suitably analyzing the resulting RF responses from the nuclei, a map or image of relative NMR responses as a function of the location of the nuclei can be determined. Following a Fourier analysis, data representing the NMR responses in space can be displayed on a CRT.
As shown in FIG. 1, the NMR imaging system typically includes a magnet 10 to impose the static magnetic field, gradient coils 14 for imposing spatially distributed magnetic fields along three orthogonal coordinates, and RF coils 15 and 16 to transmit and receive RF signals to and from the selected nuclei. The NMR signal received by the coil 16 is transmitted to a computer 19 which processes the data into an image displayed on display 24. The magnetic resonance image is composed of picture elements called "pixels." The intensity of a pixel is proportional to the NMR signal intensity of the contents of a corresponding volume element or "voxel" of the object being imaged. The computer 19 also controls the operation of RF coils 15 and gradient coils 14 through the RF amplifier/detector 22 and gradient amplifiers 20, respectively.
Each voxel of an image of the human body contains information representative of one or more tissues. The tissues of the human body are comprised primarily of fat and water. Fat and water have many hydrogen atoms (the human body is approximately 63% hydrogen atoms). Since hydrogen nuclei have a readily discernible NMR signal, magnetic resonance imaging of the human body primarily images the NMR signal from the hydrogen nuclei.
Basically, in NMR a strong static magnetic field is employed to commonly align atoms whose nuclei have a spin angular momentum and a magnetic dipole movement, i.e., atoms whose nuclei have an odd number of protons and/or neutrons. A second magnetic field, applied transverse to the first field as a single RF pulse, pumps energy into the nuclei, which causes the angular orientation of the nuclei to flip by, for example, 90.degree. or 180.degree.. After this excitation, the nuclei precess and gradually relax back into alignment with the static field. As the nuclei precess and relax, they will induce a weak but detectable electrical energy in the surrounding coils that is known as free induction decay (FID). These FID signals, as well as magnetic gradient or RF refocused "echoes" thereof, which are collectively referred to herein as MR signals, are analyzed by a computer to produce images of fat and water containing tissue; of the body.
The excitation frequency and the FID frequency are related by the Larmor relationship. This relationship states that the angular frequency, .omega..sub.0, of the precession of the nuclei is the product of the magnetic field, B.sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0.multidot..gamma.
By superimposing a linear gradient magnetic field, B.sub.Z =Z.multidot.G.sub.Z on the static uniform field, B.sub.0, which is typically defined as the Z axis, for example, nuclei in a selected X-Y plane may be excited by proper choice of the frequency of the transverse RF excitation field pulse applied along the X or Y axis. In addition, gradient magnetic fields are applied in the X-Y plane during detection of the MR signals to spatially localize emitted MR signals from the selected X-Y plane according to their frequency and phases.
As mentioned above, the main magnetic field can be altered by gradient magnetic fields created in the X, Y, and Z directions of the imaging volume. For the purpose of simplifying the descriptive mathematics, a rotating reference coordinate system X'-Y'- Z', that rotates at the nominal Larmor frequency about the Z' axis, is often used to describe nuclear phenomenon in NMR. Selected nuclei, which precess in alignment with the B.sub.0 field, are influenced (nutated) by the perpendicular magnetic field of an RF pulse at the Lamor frequency, causing a population of such nuclei to tip from the direction of the magnetic field B.sub.0. Thus, certain nuclei start aligned with the "Z'" axis by the static B.sub.0 field and then are rotated to the X'-Y' plane as a result of the RF signal pulse being imposed on them. The nuclei then precess about the Z' axis in the X'-Y' plane.
The RF nutating pulse will, of course, tip more than one species of the target isotope in a particular area. For the purpose of simplifying the description and analysis of an MRI acquisition sequence, each RF pulse used in the sequence is characterized by its center--which is representative of the time at which the nutated precessing nuclei can be considered as all being in-phase (synchronized) and after which each species of nuclei will begin to precess at their own characteristic speed. The phase of the precessing nuclei species will gradually differ (de-phase) as a result of such parameters as the physical or chemical environment in which the nuclei are located.
Once the magnetic moments of the nuclei are disturbed from their equilibrium, processes known as "relaxation" causes the Z'-component of the spinning (precessing) magnetic moments to recover to an equilibrium magnitude, M.sub.0, in alignment with the background B.sub.0 field, and the phase-coherent component in the X'-Y' plane to decay. The duration of these relaxation processes are termed the "spin-lattice relaxation" time and the "spin-spin relaxation" time and are characterized by exponentials whose defined time constants are labeled as T.sub.1 and T.sub.2, respectively. As magnetic resonance signals are exhibited through the oscillation of magnetic flux from nuclei in a plane coexistent with the X'-Y' plane, both of these relaxation processes cause the signal strength to decrease as a function of time. In the interests of accuracy, an "apparent relaxation" time constant, T.sub.2 *, is often defined as characterizing transverse signal decay due to spin-spin relaxation in the presence of B.sub.0 field inhomogeneities.
An operation whereby the various coils produces RF excitation pulses and gradient fields to result in an MR signal is called an MRI "acquisition sequence." A graphical representation of an example MRI acquisition sequence is shown in FIG. 2. In this example, the particular timing of applied RF pulses and magnetic fields is known as a "spin-echo" sequence since the MR signals appear as an "echo" of a 180.degree. spin rotation RF pulse. First, a gradient field, G.sub.slice, is superimposed along the main field to sensitize a "slice" or "slab" (for 3D imaging) of nuclei in the patient's body tissue to a particular RF resonance frequency. A 90.degree. RF excitation or "nutation" pulse, .beta., is then applied at the particular frequency to force some of the nuclei within the slab to precess in a direction perpendicular with respect to the main field. Thereafter, pulsed magnetic gradient fields of changing magnitudes, G.sub.pe (and G.sub.slice for 3D imaging), are used to phase encode the nuclei by inducing a temporary frequency difference, and hence phase differences, between nuclei in different locations along a specific direction within the slab. At the same time, a dephasing magnetic gradient field pulse 20 is applied orthogonal to the direction of G.sub.pe (shown at G.sub.read) to de-phase the precessing nuclei. Next, a "refocusing" RF pulse, .beta..sub.r, causing a 180.degree. rotation of the spinning (precessing) nuclei is applied followed by a "readout" (frequency encoding) magnetic field gradient 21 (G.sub.read) applied orthogonal to phase encoding gradient G.sub.pe. The 180.degree. refocusing RF pulse causes the spinning magnetization directions of the nuclei to at least partially re-phase--which results in producing the "spin-echo" MR signal S.
In practice, MRI sequences are almost always arranged such that the pulsed magnetic field gradients will be completely balanced by the time the center of the induced MR signal occurs (i.e., the so called "echo-center"). This is accomplished by either reversing the read-out gradient one or more times to create "field-echoes" or by applying 180.degree. spin rotation RF "refocusing" pulses to create "spin-echoes."
The time period from the center of the 90.degree. nutating pulse to the center of the spin-echo MR signal is designated as TR, the "echo time", and the entire pulse sequence duration is designated as TR, the sequence "repetition rate." Conventionally, a spin-echo sequence is "symmetric" in that the period between the 180.degree. RF pulse and the resultant echo signal is the same as the period between the 90.degree. RF pulse and the 180.degree. RF pulse.
Basically, the phase-encoding gradient field, G.sub.pe, encodes nuclei in a selected slab or slice in a first direction and the applied read-out gradient field, G.sub.read, frequency encodes the nuclei in the same slice in an orthogonal direction (also called the "readout direction"). An MR "echo" signal, S, resulting from the application of the read-out gradient field, is then acquired for many sequences, each having a unique phase encoding gradient value G.sub.pe. The acquired data (also called "raw data" or "k-space data") is then analyzed by Fourier analysis. A scaled frequency domain plot of that analysis renders information about the nuclei population in Fourier space (also referred to as the image domain), which corresponds to an X-Y-Z position.